Generally, a motor drive control system is used that drives and controls an AC (Alternating Current) motor by converting a DC power from a DC power supply into an AC power by means of an electric power converter (typically inverter). The electric power converter such as inverter converts the power through switching at a high frequency and with a high power. Therefore, the electric converter has to be configured to avoid heat generation of a switching device (high power transistor such as IGBT: Insulated Gate Bipolar Transistor for example) performing the above-described switching operation.
In particular, while a permanent-magnet-excited synchronous motor (PM motor) or the like is driven by an electric power converter such as inverter, if the rotation of the motor is locked by any external force, the electric current concentrates on the winding of one phase only, among windings of multiple phases provided to the motor. As a result, a switching device corresponding to this phase among a plurality of switching devices provided to the electric power converter (inverter) suddenly generates heat. When such sudden heat generation occurs, the switching device could be thermally broken. Therefore, a configuration is proposed for suppressing, while the motor is locked, heat generation of the switching device forming a part of the electric power converter (inverter).
For example, Japanese Patent Laying-Open No. 9-70195 (hereinafter Patent Document 1) discloses that, while an electric motor is locked, the carrier frequency of a PWM (pulse width modulation) signal is changed from a normal frequency (10 kHz) to a low frequency (1.25 kHz) to lower the switching frequency of a switching device of an inverter and thereby reduce a switching loss, so that a sudden heat generation of each switching device of the inverter is avoided.
Further, Japanese Patent Laying-Open No. 2005-117758 (hereinafter Patent Document 2) discloses a configuration in which two drive circuits are provided in parallel for each switching device of an inverter and, while only one of the switching devices is used to render the switching device on and off in a normal operation, both of the switching devices are used to render the switching device on and off in a motor-locked state. This configuration allows the switching device to be quickly rendered on and off in the locked state so that the switching loss is reduced relative to the normal state and accordingly heat generation can be reduced.
Furthermore, Japanese Patent Laying-Open No. 9-215388 (hereinafter Patent Document 3) discloses that a locked state of a motor where a certain amount of current continuously flows is detected in an early stage based on the integral of the square of the motor drive current of each phase, so as to protect an inverter.
In contrast, a configuration that is one type of the motor drive control system is disclosed for example in Japanese Patent Laying-Open No. 2003-309997 (hereinafter Patent Document 4). In the configuration, a DC voltage from a DC power supply can be stepped up by a converter and the DC voltage which is variably controlled by the converter is converted into an AC voltage by an inverter to drive and control an AC motor. In this configuration, the amplitude of the AC voltage applied to the motor can be varied according to the operational state of the motor, so that the motor can be operated at high efficiency and high power can be obtained.